When I updated my Nexus-7 (2012) tablet to brand new Android-Lollipop the tablet reaction time gets
terrible slow down.
Up to about 2 minutes to start an app, check my e-mails or get infos from the google map.
I decided to buy an other tablet, while it is so easy to get the speed from a new tablet and with installed Android 5.0 Lollipop on my Nexus-7 1.Gen. (2012) tablet.
After reset the tablet to "Factory data"  it's fast as I never imagine my Nexus-7 will ever be. Wow!
One way to get the old speed for the Android tablet or smart-phone is to reset Android to "Factory data".
The current Android version will installed and a rollback to the previews Android version is not necessary.
Of corse there are old data, libraries and files, which will get deleted with this "Factory reset" and make the Android tablet fast again.
My biggest improvement or feature in Android 5.0 Lollipop is the faster WLAN implementation. With KitKat sometimes I had problems to connect to a WLAN, what I solve with put my tablet on and off.
It was horrible!
With new Lollipop OS the WLAN connection is fast and not corrupted as it should be. Very, very good.
No worry and reasons to throw your tablet or phone in the trash. 
One other opportunity is turn on-and-off the Android device. Especially with older devices, the performance improvement is noticeable after a restart. Often there are apps running in the background and do not put yourself in a state of rest.
The specific context of the restart I don't know exactly and why it helps, but slow working devices work faster after a restart.
Now apps starts fast, switching between apps and open new browser tabs goes in a few seconds.